5 Answers
5

Rabbi Haim Vital in his introduction to Sefer Eitz Haim gives three pre-requisites to learn Kabbalah,
1) A person must be married
2) A person must have learned Gemarra for five years
3) A person must be 20 years of age.

Rav Ovadyah Hedayya(He was the Chief Kabbalist of Israel, as well as the Av Beit Din of Jerusalem, and Rav Ovadyah Yosef's Rav) in his seer De'ah V'Haskeil(1:1) states that a person who is an exceptional talmid hakham may begin younger than 20(as did his own father), though he must still be married(though it should be noted he let that requirement slide for Rav Kaduri). He also stated that a person as young as Bar Mitzvah who meets none of the requirements may, and in fact should learn Zohar.

Rav Ovadyah Yosef rules that a person needs to be married, 20, and knowledgable in the laws of Issur V'Heter(Yehave Da'at 4:47)

The Shach on YD 246:6 is by far not the final answer. Sephardi authorities do not hold by him. Neither do various Ashkenazi ones. The Gra on the same page takes him to task for not knowing enough about Kabbalah to know that the text(the Rama in the Y"D 246:6) wasn't speaking about Kabbalah. For that matter Yeshivat Sha'ar HaShamayim in Jerusalem, an Ashkenazi Kabbalistic Yeshiva, admits students to the study of Kabbalah as young as 18 and 20.
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Rabbi Michael TzadokAug 8 '10 at 23:42

Excellent source quoted by AY. Regarding Maharal and Chasidus, the danger with Kabbala is in the misinterpretation of it. There is no danger in learning kabbala pre-filtered and presented by mumchin in a ready-to-serve manner. See also the end of hakdamas haramban al haTorah.

For the sake of accuracy, Maharal and many sifrei Chassidus do not teach Kabbalah. Rather they teach insights, mussar, and other lessons or philosophical thoughts. Many if not most of what they teach is based on Kabbalah but is not actual learning of the "Hochmas HaNistar". Some Sifrei Hassidus such as Tanya or Rav Tzaddok ( and others) do also get more into actual kabbalah than others but the goal is not to teach Maaseh merkavah or Maaseh Beraishis, rather to see the Koach Hapoel Binif'al and be davek to Him.
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YahuJun 22 '10 at 2:32

Thank you Rabbi Strunk:) I based my comment partially on the Ramban that I referred to. The Ramban strongly cautions his readers against speculating about the remazim he brings as only bad will come from it (same as the pardes issue). However, he excepted the same person who learns these same areas with an attentive and perceptive ear from a "mekubal chacham". I understand this to mean he finds out the right answers from the right people and not the wrong answers from speculation. Your point is well taken that these sefarim may not be considered pardes at all.
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YDKJun 22 '10 at 5:39

Rav Ben Tzion Abba Shaul (Or LeTzion, Mussar, Shaar HaTorah, Maamar 7) writes that one should be 40 to learn Qabbala. Also, the Rokach writes in Sefer HaShem that one should 40 to learn the Qabbalistic Names of Hashem. However, the Kaf HaHaim Sofer (Orah Haim 155:12) writes that one should be twenty (see Mekubal's answer for the rest of his prerequisites).

Even though there is an opinion that one should not begin to study Kabbalah until the age of 40, the great masters of Kabbalah and Chassidut did not agree with this opinion. Some of the greatest teachers of Kabbalah--including the Ari, Rabbi Moshe Chaim Luzzatto (also known as the Ramchal), and Rebbe Nachman of Breslov--did not live to the age of 40! From an early age they began to study Kabbalah. In the Zohar we find that a sign of the coming of the Mashiach is when children will study and discuss Kabbalah.

As far as Chassidus is concerned, the Lubavitcher Rebbe said many times that this age limit referred to the time before the Baal Shem Tov. To quote from AskMoses.com

The Lubavitcher Rebbe explains that this [age limit] applied before Chassidut - the teachings of Kabbalah as prepared for the masses - was revealed to the masses by the Baal Shem Tov and Rabbi Schneur Zalman of Liadi, founder of Chabad Chassidut. During that time, the esoteric parts of Torah were considered to be a luxury, and only an elite few were privileged to be privy to Torah's inner dimension, and it was necessary for one to have exceptional knowledge and wisdom to study kabbalah.

Today, however, chassidut has been prepared for, and revealed to, everyone because it isn't a luxury anymore. Today, chassidut is necessary in order to be able to live as a G-d fearing Jew who loves and fears G-d. The longer the Galut progresses, the darker (spiritually) it becomes. In order to combat this darkness it is necessary to have the powerful light of chassidut.

Inner.org, the website of the famous contemporary kabbalist Rabbi Yitzchak Ginzburgh, says that the reason for the age limit was the concern that the knowledge of kabbalah could be misused:

The reason that some authorities have warned against studying Kabbalah at too early an age was that there were instances in Jewish history, even relatively recently, when most negative phenomena resulted from the misrepresentation and misuse of Kabbalah. For example, approximately 350 years ago, a misguided Jew, Shabbetai Tzvi, proclaimed himself the Messiah, basing himself on misinterpretations of Kabbalah. Before he was proven a fraud, he had wrought great material and spiritual suffering upon a significant portion of European Jewry.

However, it goes on to say, Chassidus is not susceptible to this problem:

This is one of the reasons that the Ba'al Shem Tov revealed a new dimension of Kabbalah--Chassidut. Chassidut expresses Kabbalah in a way that is accessible to every soul and that excludes all possibility of misinterpretation. Thus, it is highly recommended to study Kabbalah within the framework of Chassidut. When Kabbalah is studied within this framework there is no danger. If there is no danger, there is also no age barrier or other limitation on the study of the inner dimension of Torah.